The present invention relates to a method and apparatus for reducing vortex-induced-vibrations (xe2x80x9cVIVxe2x80x9d) and, more particularly, reducing VIV in marine environments by the use of helical strakes.
Production of oil and gas from offshore fields has created many unique engineering challenges. One of these challenges is dealing with effects of currents on fixed cylindrical marine elements. Such marine elements are employed in a variety of applications, including, e.g., subsea pipelines; drilling, production, import and export risers; tendons for tension leg platforms; legs for traditional fixed and for compliant platforms; other mooring elements for deepwater platforms; and, although not conventionally thought of as such, the hull structure for spar type structures. These currents cause vortexes to shed from the sides of the marine elements, inducing vibrations that can lead to the failure of the marine elements or their supports.
For short cylindrical elements that are adjacent convenient means for secure mounting, the marine elements and their supports can be made strong enough to resist significant movement by the forces created by vortex shedding. Alternatively, the marine element could be braced to change the frequency at which the element would be excited by vortex shedding.
However, strengthening or bracing becomes impractical when the application requires that the unsupported segments of marine element extend for long runs. Deepwater production risers, drilling risers, platform export risers, import risers bringing in production from satellite wells, tendons for tension leg platforms, and other conduits for produced fluids and deepwater mooring elements formed from tubular goods are typical of such applications. These pipes and tubular goods serve as marine elements in applications that are difficult or impossible to brace sufficiently to satisfactorily control vibration induced by vortex shedding. Subsea pipelines traversing valleys on the ocean floor for extended, unsupported lengths and spar hulls moored at the end of long tethers and/or mooring lines provide additional examples.
Some applications, e.g., unsupported spans of subsea pipelines, present additional challenges to deployment with onshore fabrication or topside deployment of devices for VIV suppression. In such applications it may be desirable to install the VIV suppression devices after the pipeline is layed, substantially complicating and reducing deployment options.
However, drawbacks of strakes for VIV suppression are that they have been thought to be effective only if they fully cover the circumference cylindrical structure, increasing weight and material requirements, creating handling difficulties in avoiding damaging strakes on all sides, and materially increasing the drag on the marine element.
The present invention is a partial helical strake system and method for suppressing vortex-induced vibration of a substantially cylindrical marine element, the strake system having a base connected to the cylindrical structure and an array of helical strakes projecting from the base for about half or less of the circumference of the cylindrical structure.